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Water shortage killed off island mammoths

US study shows how current island-dwelling animal populations may also suffer the effects of rising sea levels. Amy Middleton reports.

Some of the last woolly mammoths, surviving on an Alaskan island thousands of years after mainland populations disappeared, were driven to extinction as their drinking water sources dried up thanks to climate change, a new study suggests.

Russell Graham at Pennsylvania State University and a team of US colleagues analysed thousands of years of sediment layers from the bottom of a freshwater lake. The presence (and absence) of mammoth DNA and organisms suggests a combination of rising sea levels and shortage of freshwater finished the population of woolly mammoths off.

The woolly mammoths on St Paul Island, one of five volcanic islands off the coast of Alaska, in the Bering Sea, were among the last on Earth.

While most woolly mammoths died off between 14,000 and 10,000 years ago, a population survived on St Paul Island until 5,600 years ago.

"Freshwater resources look like the smoking gun for what pushed them into this untenable situation."

Human hunters didn’t kill them off – while modern humans co-existed with woolly mammoths in some parts of the world, on St Paul Island, there’s no evidence of humans until Russian whalers arrived around 1787 BC.

So the research team went back in time and analysed ancient sediment cores dug from a freshwater lake near the middle of the island.

Among the sediment layers, trapping bits of fungal, plant and animal matter over the millennia, the team identified mammoth DNA and fungi that grows on animal faeces. Both declined around 5,600 years ago.

Small crustaceans and plankton trapped in the sediment showed the mammoths’ decline coincided with a significant shallowing of the island’s freshwater lakes.

And analysis of nitrogen in mammoth bones and teeth showed progressively drier conditions leading to their extinction.

The researchers believe as sea levels rose, mammoths would have been competing for diminishing freshwater and less land area on the island.

“Multiple reinforcing lines of evidence indicate that reduced freshwater availability triggered the extinction of St Paul mammoths at 5,600 [years ago],” the paper reads. This inferred timeline represents one of the most precise estimates for a species’ extinction on record.

"It paints a dire picture of the situation for these mammoths," says co-author Matthew Wooller at the University of Alaska Fairbanks.

"Freshwater resources look like the smoking gun for what pushed them into this untenable situation."

The results are particularly relevant today, the researchers write, because they support evidence of the unique conditions that will be faced by island-dwelling populations in times of intense climate change.